Fix entitlement tools to build (elastic#117351)

This commit adjusts the common lib of entitlement tools to use
elasticsearch.build so that it gets java version configuration
automatically. Additionally the mrjar plugin is removed from the core
lib since it is not used there.

libs/core/build.gradle

@@ -8,7 +8,6 @@
  */
 
 apply plugin: 'elasticsearch.publish'
-apply plugin: 'elasticsearch.mrjar'
 
 dependencies {
   // This dependency is used only by :libs:core for null-checking interop with other tools

libs/entitlement/tools/common/build.gradle

@@ -7,9 +7,8 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-plugins {
-  id 'java'
-}
-
-group = 'org.elasticsearch.entitlement.tools'
+apply plugin: 'elasticsearch.build'
 
+tasks.named('forbiddenApisMain').configure {
+  replaceSignatureFiles 'jdk-signatures'
+}